Jurassic Park or the World of Cycads

Cycads are very primitive plants, and their association with beetles and other insects should also be very old. This association is of special interest to those entomologists who are trying to make out palaeohistory of insects.

Cycads are ligneous, palmiform plants, with pennate fronds, i.e. leaves with leaflets, and they produce seeds. They resemble palm trees, but are not at all related to them, and are much more primitive. They are classified among the Gymnospermae, together with the Ginkgoales and the Gnetales, which also do not look like Coniferae. Like Gnetales, they are very toxic and they concentrate a great part of that toxicity into the cones. It seems that beetles feeding on the cones and the leaves do that partly for pharmacophagy. Cycads constitute a unique group of plants, very ancient, probably dating from the Permian, along with the precursors of flowering plants. Cycads had their maximum development during the Mesozoic. They all produce male and female cones, the sexes being separate (dioecism). This production of cones can be annual or spaced out by several years. Coevolution between beetles and cones remains sometimes difficult to understand, the Coleoptera having generally an annual cycle.

Cycads are very primitive, and are among the actual seed plants. They are practically unique, sharing this feature only with the Ginkgos, in producing mobile male cells. They also represent an important link with the old seed plants, the Pteridospermeae and the Bennetitales. Cycadales were flourishing during the Mesozoic (-206 to -65 myr), and they were in this period, mostly during the Jurassic and the Cretaceous, very largely distributed over the planet. They were then much more varied than the present survivors. Cycads are actually relics, with a disjunct distribution, essentally tropical, with the exception of Japan and Florida.

Cycadales at present are a small group, consisting of around 185 species and 11 genera (Jones, 1993; Norstog and Nicholls, 1997). But the number is increasing with the intensity of the taxonomical researches (Schutzman, 1984, 1987, 1989). There are certainly synonyms, but many species remain undescribed. The real number may approach 210 species, all archaic survivors of once a large group, which covered the planet, with the Bennetitales, during the time of the dinosaurs and the Archeopteryx.

Among all those genera, the American Zamia shows an incredible variability, since it can be arborescent, subterranean, epiphyte, myrmeco-phile, or cremnophile, i.e. adapted to abrupt cliffs. It shows also all sizes from the minuscule Zamia pygmaea Sims from Cuba, actually endangered, up to the enormous Z. lindenii Regel from Ecuador and Peru. Zamia's preferences vary enormously, from the cloud forest in mountains, up to the mangrove, the plain forest, and the savannah. The genus lives as well on sandy soil as on red ferrugineous laterite, on rich humus, in plains and in dense forests, on the cliffs or along the torrents. Zamia species can be arborescent trees as well as plants with hypogean roots. The biggest density of the group is in Central America. Hybrids exist, and have contributed to the general evolution of the genus.

A peculiarity of the Cycads is that some individuals may suddenly undergo change of sex. Another abundant group during the Mesozoic was the Bennetitales. They disappeared by the end of the Cretaceous, but cycads survived. Relationships of these fossil plants with insects are proven, mostly with the beetles. However, Bennetitales had bisexual cones, surrounded with petaliform bracts, sort of floral corolla. Those plants did not possess male or female individuals like the present Cycads and cross pollination was probably not a problem with them. Bennetitales did not have "flowers" on the trunk, as shown in the old drawings, but the cones, which were closed, except perhaps in Williamsonia with an arboreal form. The insects visiting those cones were necessarily borers, and we think that it was that way that they fertilized those plants. Cycad seeds, as probably also in case of now extinct forms, are disseminated by the mammals, the birds, the reptiles and by water. Probably during the Mesozoic, the big reptiles were important disseminators.

Cycad cones produce, during certain periods of the day (Tang, 1989; Tang et al, 1987; Terry et al, 2004), volatile components and various odours, resinous, fruity, mouldy, etc., certainly to attract pollinating insects. The cones are visited by more or less specific beetles, and the young fronds by Lepidoptera, mostly lycaenids, and many beetles. We suppose that the beetles attracted by male cones and their nutritive sources go later on to female cones by "mistake", and in doing so pollinate them. It, however, remains to be adequately demonstrated for certain species of beetles and cycads, because female cones are generally very tightly closed, and are often without insects within them. That is why anemophily (i.e. dissemination by wind) was, until mid-1980s, supposed to be the only fertilizing mechanism for the cycads. Beetle intervention is now fully proven, for example the Australian cycad, Lepido%amia peroffskyana (Zamiaceae), which is pollinated exclusively by Tranes weevils (Hall et al, 2004). Many supporters of the coleopterous fertilization have written about it (Nor-stog et al\ 1992; Rattray, 1913, Tang, 1987a and b; Jones, 1993; Norstog and Nicholls, 1997). Crowson mainly (1981, 1989, 1991), in his remarkable study of the relationships beetles-cycads, has strongly insisted on the pollinating role of the beetles. Some Maro%amia in Australia are pollinated only by some species of thrips (Terry et al, 2004).